1. Field of the Invention
This invention relates to the field of pulsed-doppler sonars and their application to navigation and oceanography.
2. Description of the Prior Art
The current vectors of deep water masses in a body of water such as the ocean are customarily measured by current meters. The meters are attached at various depths to the anchor cable of a taut-line buoy. To determine the currents over a wide area, a large number of such buoys are required. The necessary number and their spacing depends, of course, upon the degree of resolution desired. Since the buoys with their attached current meters are fixed in place, the arrangement is somewhat inflexible. Another method employing free-fall bouys is disclosed in U.S. Pat. No. 3,972,231.
Greater flexibility could be attached if the currents were measured from shipboard. The ship could easily and quickly cover a very wide area. Surface currents of course, are determined by noting the corrections necessary to hold a ship to a desired track and speed. The term "track" is defined as the actual course of the ship over the earth's surface as opposed to "heading" which defines the direction in which the bow of the ship is pointing. The track vector defines the direction and speed of the ship along the desired course. Because wind also affects the ship's heading, the heading correction is not necessarily a true measure of the surface currents.
Near-surface currents can be measured by noting the deflection of a long towed streamer, relative to the ship's track. One such system, designed for seismic exploration, is described in co-pending U.S. patent application Ser. No. 740,167 and now abandoned, assigned to the assignee of this invention. In the above system, a hydrophone streamer cable more than two miles long is towed behind the seismic ship. Embedded in the streamer cable, along with the hydrophones, are a number of sonar-like sensors. Two transmitters or pingers are mounted on outriggers on each side of the ship, along a base line. Using the difference in arrival times of the "pings" at each sensor, it is possible to compute by triangulation, the amount and direction of the streamer deflection. Streamer deflection can be translated into a current vector for near-surface currents.
Determination of deep ocean-current profiles is fraught with many difficulties. In fact, William VonArx devotes an entire chapter, chapter 8, to current-measurement problems in his book "An Introduction to Physical Oceanography", Library of Congress Catalog No. 61-5026. Accordingly an improvement in the art would be welcome to oceanographers.
Many geographical exploration ships now carry multisensor, integrated navigation systems. The many sensors provide redundancy such that if one sensor becomes inoperative, the other units will hold the ship to the desired track. Such systems may employ a pulsed doppler sonar, satellite receiver, inertial platform, and various radio navigation devices such as VLF, Loran C, Shoran, Decca etc. One such integrated system is disclosed in U.S. Pat. No. 3,630,079.
As is well known in navigational circles, a pulsed doppler sonar in combination with a high quality gyro compass, provides a very good means for determining the ship's track vector. But the above statement is true only provided the doppler sonar pulses are locked on the water bottom. If the pulses lock on a water mass at some depth, the indicated ship's track will be in error by the amount of the water-mass current vector. Heretofore, it was considered a nuisance when the doppler sonar locked on a water mass instead of on the water bottom. Every effort was made to avoid water-lock. In U.S. Pat. No. 3,437,987, the inventor went to the trouble of mounting the doppler sonar unit in a "fish" that was towed behind the ship at an altitude of one or two hundred feet above the sea floor to insure that the sonar would always track on the bottom rather than on a water mass.
It is an object of this invention to make use of the ability of the doppler sonar to lock onto a water mass and, using others of the sensors of the multisensor navigation system to maintain the desired ship's track, to determine the current vector of a water mass. As far as I am aware, productive use of the "nuisance" capability of the doppler sonar has not previously been suggested in the prior art.
U.S. Pat. Nos. 3,257,638, 3,491,333, and 3,617,995 are incorporated herein by reference as disclosing the operation of typical prior-art doppler sonar navigation systems.